The present invention relates to negative electrodes for electrochemical generators, more particularly to negative electrodes made out of cadmium or zinc for generators in an alkaline, neutral or acid medium.
As regards the cadmium electrodes used substantially as negative plates in alkaline nickel-cadmium and silver-cadmium generators, it is known how to prepare them by means of three types of processes (the third of which is also applicable to the zinc electrodes), namely,
1. The process whereby a conductive porous support or core is repeatedly immersed, consisting in immersing a support, generally in sintered nickel, in melted cadmium nitrate, and thereafter in an aqueous solution of an alkaline metal hydroxide which transforms said nitrate in cadmium hydroxide. The pores of the cadmium hydroxide conductive support forming the active material are thereby filled up. Of course, the two immersion operations are repeated several times for providing a sufficient deposit of active material so that the electrode reaches its required capacity, that is, in operation, is capable of storing the required electricity quantity.
Excellent cadmium electrodes are thereby obtained, but their cost of production is very high.
2. The process whereby a non-porous conductive support or core is coated with a powder and consisting in coating, for instance, through compacting or impasting with a cadmium base and/or a cadmium compound powder, a non porous conductive support made, for instance, of a metallic gauze or a perforated plate.
This second process is more economical than the first, but the electrodes which it provides cannot support high loading or unloading conditions and their thickness cannot be very thin, which is often a disadvantage.
3. The process whereby cadmium and/or cadmium hydroxide is directly electro-deposited on a support or core, porous or non porous, consisting in cathodically polarizing said support while immersing it in a solution of at least one cadmium salt.
Such a process provides at a relatively low cost, good quality cadmium electrodes withstanding high ratings, such as the electrodes obtained through the first process, and which may be very thin.
Generally, according to how the electrodeposition is being performed, three types of electrolytic metallic deposits may be made:
(A) AN ADHERENT DEPOSIT, COMPACT AND WITHOUT POROSITIES, OBTAINABLE THROUGH CONVENTIONAL ELECTROPLATING; IT SHOULD BE NOTED THAT IT IS ALSO POSSIBLE TO OBTAIN A ZINC DEPOSIT OF THIS TYPE; SUCH CADMIUM OR ZINC DEPOSITS ARE CONVENIENT FOR PROVIDING PROTECTIVE PLATINGS, BUT NOT NEGATIVE ELECTRODES, SINCE THEY RESUME THEIR PASSIVITY AS THE METAL IS NOT IN A DIVIDED STATE;
(B) AN ELECTROLYTIC DEPOSIT, ADHERENT AND POROUS, USABLE FOR MAKING ELECTRODES; A PROCESS OF THIS TYPE, WHICH IS THE OBJECT OF French Pat. No. 1,281,247 filed on July 6, 1960 by YARDEY INTERNATIONAL CORPORATON, consists in electrolytically depositing an active metal on the cathodic surface from a highly ionized solution containing a large proportion of ions others than those of said active metal, the active metal ions being present in solution in a proportion lower than about 10% by weight; the active metal can be cadmium or zinc for forming negative electrodes, or silver for forming positive electrodes; the porous deposits on electrodes, in particular on negative electrodes, obtained by a process of such type offer the disadvantage of having a far lower reactive power than deposits in powder form obtained by other processes and, moreover, this reactive power decreases after repeated cyclings; therefore, they do not provide very good quality electrodes;
(c) a powdery deposit, of fine texture and little adherent, usable for preparing metallic powders; such powders are very reactive since very divided, but the deposit offers the disadvantage of being little adherent, heterogeneous and fragile, whereby it cannot be used for forming the active part of an electrode.